In order to meet stringent exhaust emission regulations, some engine manufacturers have installed exhaust after-treatment systems comprising, in part, of particulate traps (“filters”). These filters collect soot—a mixture of carbon particulates and condensed organic material—and any inorganic particulates (“ash”), which are produced primarily as a result of the combustion of small amounts of engine lubricating oil. The concentration of the soot collected in the filter is constantly monitored for the purpose of signaling to the engine controller when regeneration, or cleaning, of the filter is necessary. Regeneration of the filter is necessary in order to reduce the exhaust backpressure to the engine and to protect the filter from damage.
All filter regeneration strategies involve some sort of soot oxidation process that must be carefully managed in order to avoid thermal damage to the filter. In particular, the carbon particulates contain the highest calorific content and, as a result, carbon particulates release the highest amount of energy within the filter during the regeneration process. Therefore, it is essential to accurately assess the carbon particulate concentration within the filter prior to initiating the filter regeneration process.
To date, one of the primary methods for assessing soot accumulation within the filter has been to measure the pressure drop across the filter. Due to the large number of engine operating parameters that effect engine exhaust flow rates—and thus pressure drop across a filter—any correlation between pressure drop and soot concentration may not accurately determine particulate trap loading. The resistance-to-flow by soot is also a function of the ratio of the carbon particulates to condensed organic concentrations—the condensed organic material is often referred to as the soluble organic fraction (“SOF”)—which is difficult if not impossible to determine by pressure drop measurements. Similarly, some engine manufacturers have found it difficult to differentiate between accumulated soot and ash in a filter by pressure drop across a filter.
This application discloses, among other things, a system that uses an RF-based measurement method to directly measure carbon particulate concentrations within the filter. After filter regeneration, the method can also be used to measure ash build up within the filter.
To those knowledgeable in the art, it is well known that the transmission of an RF signal through a non-magnetic medium is effected by its complex permittivity. The real component is called the dielectric constant and the imaginary component is called the loss factor. The dielectric constant affects the space velocity of an RF signal and loss factor is essentially a resistive component that converts RF energy to heat. The permittivity of a medium is a function of its atomic structure and density, and can vary with temperature and RF frequency. Differences in the permittivity of materials form the underlying basis for RF-based measurement methods.
Ceramic filters, made of materials such as cordierite or alumina, are largely transparent to RF energy. That is, these latter ceramic materials have a very low loss factor. In contrast, carbon particulates have a relatively high loss factor and are hence a good absorber of RF energy. The accumulation of carbon particulates effectively alters the apparent permittivity of a ceramic filter. U.S. Pat. No. 5,497,099 to Frank Walton (“'099”) discloses that it is possible to monitor the level of soot accumulation on a diesel engine filter medium by detecting changes in the effective permittivity of the ceramic filter medium
As further disclosed in '099, an antenna system comprising of parallel transmitting and receiving antennae is inserted parallel to the central axis of the cylindrical metal filter cavity and is inductively coupled in a direction radial to the antennae and filter axis. These antennae may be inserted in either opposite ends of the filter or within the same end of the filter. It can be readily demonstrated that the measurement volume is axially confined to the area of overlap of the antennae and in the radial direction by the metal walls of the metal filter housing. Each antenna may consist of one or more metallic elements. The addition of more than one element to an antenna may be in some applications to improve the broadband frequency transmission and reception characteristics of the antenna system. An amplitude modulated RF source sends a signal to a splitter. The splitter applies the signal to both the transmitting antenna (20) and to a detector. This latter detector produces a reference output signal that is representative of the power of the signal prior to transmission to the transmitting antenna (20).
Further referencing '099, a second detector is provided, which is electrically connected to the receiving antenna, and which produces an output signal representative of the power transmitted through the filter medium. The first and second detector output signals are applied to a comparator that produces an output signal, which is proportional to the difference in the signal strength of the transmitted and received signals. Accordingly, the transmission loss through the filter medium, which, in turn, is representative of the change in the effective loss factor caused by accumulation of soot within the filter. That is when there is little or no accumulation of soot in the filter there will only be a small transmission loss in the signal strength. As the soot accumulation increases, the difference in signal strength between the transmitted and received increases. The comparator can be designed to provide a variable output that is a function of the soot accumulation within the filter medium or to indicate when a certain predetermined soot level is reached, or both.
'099 further provides that the power source is arranged to emit RF energy over range of frequencies with the preferred frequency band being up to one octave, i.e., a 2 to 1 range in frequency. The signal is averaged over a preferred bandwidth.
'099, however, fails to differentiate between the relative concentrations of carbon particulates and SOF. High levels of SOF interfere with the ability of the RF sensor system to accurately assess the carbon particulate concentrations within a filter. The system disclosed in '099 fails to provide sufficient RF measurement parameters to differentiate between variable concentrations of carbon particulates and SOF. The system disclosed in '099 also fails to provide a method for assessing ash accumulation within the filter medium.
The system of the present disclosure measures both transmitted and reflected RF power over a range of discrete frequencies. In effect, the RF spectral response can be shown to uniquely characterize both the quantity and the composition of the soot, i.e., the ratio of the carbon particulates to the SOF. This additional RF spectral information can be used to develop correlations.
In addition to being able to determine the accumulation of soot in a filter, the disclosed system provides for a method of determining the accumulation of ash in the filter. Measurements of the complex permittivity of ash indicate that the loss factor is very low under the temperature conditions where soot is being filtered from diesel exhaust. That is, it does not interfere with the ability to detect soot accumulation. However, at temperatures at or above where soot oxidation occurs, the loss factor increases with increasing temperature. Hence after the soot has been removed by oxidation and the filter remains at regeneration temperatures, the effective permittivity of the filter reflects the thermally enhanced permittivity of the ash. It is, therefore, possible by development correlations to determine the accumulation of ash within the filter. As with the determination of soot, both reflected and transmitted power measurements over a range of frequencies can be used to develop these latter correlations.